Disable ClockCache db_crashtest

Summary: Tempororily disable clock cache in db_crashtest while we investigate data race issue with clock cache.

Test Plan:
    python ./tools/db_crashtest.py blackbox

Reviewers: sdong, lightmark, IslamAbdelRahman

Reviewed By: IslamAbdelRahman

Subscribers: andrewkr, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D62481

tools/db_crashtest.py

@@ -19,7 +19,7 @@ import argparse
 default_params = {
     "block_size": 16384,
     "cache_size": 1048576,
-    "use_clock_cache": lambda: random.choice(["true", "false"]),
+    "use_clock_cache": "false",
     "delpercent": 5,
     "destroy_db_initially": 0,
     "disable_data_sync": 0,
@@ -85,7 +85,7 @@ whitebox_default_params = {
 simple_default_params = {
     "block_size": 16384,
     "cache_size": 1048576,
-    "use_clock_cache": lambda: random.choice(["true", "false"]),
+    "use_clock_cache": "false",
     "column_families": 1,
     "delpercent": 5,
     "destroy_db_initially": 0,